1. Technical Field
The present invention relates to an elasticized web comprising a gatherable substrate and a multi-strand elastic yarn for use primarily, though not exclusively, in absorbent articles such as diapers and incontinence garments. The invention further relates to a method and apparatus for manufacturing an elasticized web.
2. Background of the Invention
It is known to produce elasticized webs in a number of different ways. Traditionally, an elastic yarn is elongated under a tensioning force and is affixed using adhesive to a non-tensioned substrate which is to be gathered. The adhesive may be applied either to the elongated elastic yarn at discrete locations along the yarn before the elongated yarn is made to contact the substrate which is to be gathered, or the adhesive may be applied in a predetermined pattern, such as in a grid or helical fashion, to the substrate. Alternatively, the adhesive may be applied to both. Irrespective of how and where the adhesive is applied, the elongated yarn bonds to the substrate at discrete locations and, when the tensioning force is removed, the elastic yarn tends to contract back towards its original, non-tensioned length, thereby gathering the substrate.
While the above-described method has been widely adopted, it suffers from several drawbacks. Firstly, due to its inherent stickiness, the use of any adhesive in a production process requires that the adhesive be accurately metered at exact locations, otherwise components of the production machinery, as well as undesired regions of the article of manufacture, may become contaminated by the adhesive. Secondly, in fluid permeable articles, adhesive tends to reduce the breathability of the article, something which is clearly disadvantageous in articles such as disposable diapers. Thirdly, adhesive acts as a barrier to the transport of liquid, again possibly to the detriment of article such as disposable diapers. Fourthly, adhesive tends to stiffen an article, i.e. those regions of an article in which adhesive is present are less flexible than those regions having no adhesive. Fifthly, adhesive is an additional component which must be procured and handled, thereby contributing to the production costs of the manufactured article.
In an attempt to overcome at least some of these drawbacks, a method of producing an elasticized web without the use of adhesive is disclosed in WO-A-97/34506. According to this document, an elastic yarn passes in a stretched condition through a sleeve formed by a substrate which is to be gathered. At spaced locations along the yarn, connecting portions are formed in the sleeve on radially opposed sides of the yarn. The connecting portions define a passage of restricted size through which the elastic yarn in its stretched condition can pass due to its decreased diameter in the stretched condition. These connection portions are produced by passing the sleeve and stretched elastic yarn between a pair of rollers. One roller is provided about its periphery with a plurality of sets of axially opposed raised heels. The gap between the sets of axially opposed raised heels defines a peripheral channel with which the elastic yarn is aligned. The heels are arranged to generate heat, for example by ultrasound, such that, as each set of opposed heels contacts the sleeve formed by the substrate, heat fusion of the material of the sleeve takes place, thereby forming the connection portions. Once tension in the yarn is released, the diameter of the yarn tends to revert to its non-tensioned dimension. However, those portions of the yarn in the passages between the connecting portions are unable to expand due to the restriction created by the opposed connecting portions. The radial expansion of the yarn either side of each pair of connecting portions effectively immobilizes the yarn in the connecting portions. Thus, as the yarn contracts in the axial direction, it gathers the substrate to thereby create an elasticized web.
Although the elasticized web produced in accordance with WO-A-97/34506 does not suffer from the drawbacks associated with adhesively attached elastic yarns, its method of manufacture nevertheless makes it unattractive for certain mass production applications, particularly for disposable absorbent articles. A major disadvantage is the need for very close tolerances to create the passage of restricted size between the opposed connecting portions. A further disadvantage is that, for each different diameter of yarn which is to be employed, a corresponding pair of rollers is required. Furthermore, since the diameter of an elastic yarn is dependent on the degree of elongation of the yarn, a separate pair of rollers is required for each desired amount of elongation of one and the same elastic yarn. In addition, the method according to WO-A-97/34506 is not easily adaptable for products having a plurality of relatively closely spaced elastic yarns. This is due to the fact that each yarn must be located in its own peripheral channel during manufacture of the elasticized web. In addition, only rectilinear lines of connection would seem to be able to be formed using the above-described method, thereby excluding the possibility of laying out the elastic yarn in a curved path. Finally, the connection between the elastic yarn and the substrate relies entirely on friction.